The invention relates to an apparatus and method for generating harmonics in an audio signal.
Methods and circuits for generating harmonics are used in devices for acoustic reproduction, such as television receivers, radio receivers, and stereo systems, to compensate the frequency response of the loudspeakers, in order to improve acoustic reproduction and also to prevent the device or the system from being overdriven.
A critical element in a device for acoustic reproduction is the loudspeaker, whose acoustic pressure, below a structure-based limit frequency, drops about 40 db per decade. This corresponds to the transmission function of a second-order filter. On the other hand, bass reflex and transmission line loudspeakers have transmission functions corresponding to a filter of higher order. The lower limit frequency typically lies between about 50 Hz and 200 Hz. The lower the limit frequency of a loudspeaker, the more expensive it is to produce. Consequently, economical devices such as TV sets and portable radio receivers are equipped with simpler loudspeakers, whose lower limit frequency is relatively high. To improve acoustic reproduction in the lower frequency range, the limit frequency is displaced downward by pre-amplifying the low frequencies. However, this can cause the final amplifier and the loudspeakers to be overdriven. To prevent the final amplifiers or loudspeakers from being overdriven and possibly even being destroyed, the output signal of the bass amplifier is fed back in such a way that the amplification of the lower frequencies is reduced for a large output signal. For example, U.S. Pat. No. 5,305,388 entitled xe2x80x9cBass Compensation Circuits For Use In Sound Reproduction Devicexe2x80x9d discloses such a method.
U.S. Pat. No. 5,359,665 entitled xe2x80x9cAudio Bass Frequency Enhancementxe2x80x9d discloses a circuit arrangement in which the audio signal is conducted via a first path directly to the first input of an adder, and conducted via a second path through a low-pass filter and an amplifier with variable amplification to a second input of the adder. The output of the amplifier is fed back via a signal level detector to its control input. This measure reduces overdrive of the final amplifier.
From psychoacoustics, it is known that a person can still clearly determine the fundamental frequency of a tone even if the fundamental frequency is not present in the spectrum, but only harmonics of the fundamental frequency. This psychoacoustic effect is utilized in that harmonics of the fundamental frequency are generated and are conducted to a loudspeaker whose limit frequency lies above this fundamental frequency. A listener thus thinks that he is hearing this low fundamental frequency, even though the loudspeaker does not radiate it. For example, a listener thinks that he is hearing a tone of 50 Hz when the loudspeaker in fact does not transmit this low tone, but only a tone of 250 Hz and a tone of 300 Hz. Advantageously, a listener subjectively perceives the difference of 50 Hz.
This effect is utilized when qualitatively simple loudspeakers with a high lower limit frequency of for example 120 Hz, are supposed to transmit for example a signal of 60 Hz. One then generates harmonics of the 60 Hz signal whose difference amounts to 60 Hz. The listener then actually thinks he is hearing a 60 Hz tone, although this tone is not radiated by the loudspeaker.
To generate the harmonics electronic circuits are required to determine the fundamental frequency in a mixture of audio signals, and to extract it in order to generate the harmonics of this fundamental frequency.
U.S. Pat. Nos. 5,668,885 and 5,771,296 describe the generation of harmonics by forming absolute values by a rectifier arrangement.
U.S. Pat. Nos. 4,150,253 and 4,700,390 describe the generation of harmonics by clipping (i.e., cutting off the amplitude of the fundamental frequency of the audio signal above a certain level).
In all these documents, filters with fixed corner frequencies are used to select the signals whose harmonics are to be generated.
If there is more than one signal in the selected frequency range (as is the case for real audio signals), which are composed of a frequency spectrum, then not only the harmonics of the existing signals are generated but also harmonics with an undesirable frequency, which consist of the sum of all the existing signal frequencies and their multiples. The result of this is that the tone finally radiated by the loudspeaker sounds very muddy.
Therefore, there is a need for an improved technique for generating harmonics.
Briefly, according to an aspect of the present invention, a system for generating harmonics in an audio signal includes a filtering device that is responsive to an audio input signal, and determines a dominant fundamental frequency component within the audio input signal and provides a filtered audio signal indicative thereof. A non-linear unit (e.g., a multiplier) responsive to the filtered audio signal generates harmonic frequency components of the dominant fundamental frequency and provides a non-linear unit output signal indicative thereof to a first bandpass filter that provides a first bandpass filtered signal. A summer sums the audio input signal and the first bandpass filtered signal to provide a system output signal that includes harmonics of the dominant frequency component.
In one embodiment, a system delimits as precisely as possible the signal frequency whose harmonics are to be generated with a variable filter (e.g., a band-pass filter).
A second embodiment generates harmonics by using the nth power (e.g., two) of the input signal to generate the (nxe2x88x921) harmonic. If n is equal to two, the fundamental frequency of the input signal is squared and the first harmonic is generated. Fundamental frequency is here understood to designate the dominating frequency contained in the audio signal, within a frequency range of for example less than 120 Hz. Generating harmonics by potentiation is much xe2x80x9ccleanerxe2x80x9d compared to the known clipping or rectifying process. The amplitude of the signal is corrected before or after potentiation.
Advantageously, a system of the present invention creates an improved audio impression for a listener when he hears an audio signal that is radiated by loudspeakers with a relatively high lower corner frequency.
These and other objects, features and advantages of the present invention will become apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings